Conventionally, according to a known steam supply system of the above-noted type, a steam supply passage incorporates a pressure reducing valve and the passage further incorporates a steam ejector downstream of the pressure reducing valve. A suction portion of the steam ejector is connected to a re-evaporation tank for re-evaporating steam condensate via a suction passage. Passage steam of the pressure reducing valve is used as a driving steam for the steam ejector. In operation, re-evaporated steam within the re-evaporation tank is suctioned by the steam ejector to be mixed with the passage steam. The suction passage incorporates a check valve for preventing reverse flow of steam to the re-evaporation tank (see, e.g. Patent Document 1 identified below).
Namely, this steam supply system is configured to achieve energy saving of the entire plant such as a steam plant by feeding re-evaporated steam present inside the re-evaporation tank to the steam-using device through the suction function of the steam ejector incorporated in the steam supply passage.
And, in the conventional technique, as an arrangement of allowing quick passage of re-evaporated steam for immediate realization of a valve closed state leads to reliable prevention of reverse flow, the check valve incorporated in the suction passage is provided with such inherent flow characteristics that an increase ratio of a flow rate associated with increase of its valve opening ratio is greater in a small opening ratio range with small valve opening ratios than in a large opening ratio range with large valve opening ratios (the inherent flow characteristics indicated by (a) in FIG. 2). For example, the valve is configured as shown in FIG. 8 in which a valve body 43 has a disc-like valve lid portion 42 whose face 42a on the side of a valve opening 41 is provided as a flat face. Meanwhile, numeral 44 denotes an inlet passage, numeral 45 denotes an outlet passage, numeral 46 denotes a compression coil spring (an example of the “urging means”), numeral 47 denotes a valve chamber, and numeral 48 denotes an annular valve seat. Also, FIG. 8 (a) shows a valve closed state and FIG. 8 (b) shows a valve opened state providing a flow rate (specifically a Cv value to be described later) of about 20%.
In the check valve shown in this FIG. 8, in association with a movement of the valve body 43 due to a fluid pressure of a fluid, the area of an annular gap A1 formed between a valve seat contacting portion (the outer peripheral portion of a bottom face 42a of the valve lid 42) of the valve body 43 and the annular valve seat 48 becomes the minimal area of the inlet passage of the fluid. Hence, the passing flow rate of the fluid is determined by the area of the annular gap A1 at the time of valve opening. As the increase ratio of the area of the gap A1 associated with increase in the valve opening ratio based on the amount of movement (stroke) of the valve body 43 away from the valve seat 48 is progressively decreased in association with increase of the area of the gap A1 in response to increase in the valve opening ratio. Hence, the check valve has the inherent flow characteristics of the type shown by (a) of FIG. 2.
Incidentally, in recent years, there are growing concurrent needs for further enhancement of energy saving effect of the whole plant through effective utilization of even a very small amount of re-evaporated steam by the suction function of the steam ejector and for accurate control of e.g. steam to be supplied to a steam-using device in response to various situations relating to e.g. the type of the steam-using device or the type of the plant including this device or a desire of the plant manager. And, the supplying steam for this steam-using device or the like is controlled by adjustment of the suction pressure of the steam ejector through adjustment of the opening ratio of the pressure reducing valve.